This invention relates to a heat-sensitive recording material. More particularly, the invention relates to a heat-sensitive material including a solvent resistant and friction reducing protective layer useful in the manufacture of adhesive-backed heat-sensitive labels and other thermally sensitive paper stock used in printers and facsimile equipment. The labels are useful in packaging goods which, in transit, storage, or display, may be exposed to diverse solvents, and may be used on meat, produce, or articles of manufacture commonly exposed to water or oleophilic materials. A bar code and/or alphanumeric information may be formed on such labels at the point of sale by imaging the label with a thermal print head.
Known recording materials have a thermally imageable layer comprising a binder, a colorless or pale leuco dye, and an acidic substance that causes the dye to change color on the application of heat. Labels made from such materials are commonly used in grocery stores, delicatessens, and other points of retail sale of commodities sold by weight. Increasingly, they are also used on many other products. At or prior to a sale, the retailer weighs the product, commonly on a machine which integrates a scale, register, and thermal print head, and actuates the machine to deliver a thermally imprinted label indicating the price, weight, and other information in coded and/or alphanumeric form. The label is then affixed to the product, typically by means of a pressure-sensitive adhesive backing layer.
Labels of this type are often exposed to water, fats, oils, and other solvents which can have an adverse effect on the thermal image, increase background discoloration, and in some cases, destroy the machine readability of the imprinted bar codes. The labels are often supplied in strips which may be serially printed rapidly. As printing speed increases, the labels have been observed to jam about the printhead, requiring shutdown, cleanout, and restart.
The risk of exposure to deleterious solvents is much lower in paper intended for use in thermal printers and facsimile devices, but image stability and thermal head-paper frictional effects are nevertheless important in such products.
The use of protective coatings on thermally sensitive materials to protect the thermal image from the deleterious effects of solvents is known. U.S. Pat. No. 4,388,362 to Iwata et al. teaches the application of a water-soluble, resinous protective coat over the heat-sensitive layer. See also U.S. Pat. No. 4,370,370. U.S. Pat. No. 4,591,887 to Arbree et al. teaches the deposition of a resinous protective layer which is covalently cross-linked by melamine formaldehyde in situ to impart very significantly improved solvent resistance. However, all of these protective layers are subject, in varying degrees, to penetration by solvents because of the character of the materials from which they are made.
Previous attempts also have been made to decrease the friction between the printhead and recording material through the use of chemical additives and polymer films. Japanese Patent Application No. 60-129295-A is understood to disclose a heat transfer material comprising a polyethylene derivative polymer and a silicon or fluorine based surfactant or lubricant. Japanese Patent Application No. 60-094390-A is understood to disclose a backing layer including a fluorinated surfactant designed to inhibit sticking of the printing material to the printhead. Japanese Patent Application No. 60040293-A is understood to disclose a heat transfer material including a film containing a lubricant such as a fluorine-containing compound which prevents thermal head sticking.
The chemical engineering involved in designing a barrier layer which not only protects the imaged underlayer from damage from commonly encountered solvents but also overcomes friction at the printhead and consequent jamming can be particularly difficult. It is accordingly an object of this invention to provide heat-sensitive recording material whose thermal image is protected from discoloration, and which is characterized by significantly lower friction between the printhead and recording material.